The Black Douglas is a historical fantasy novel written by Samuel Rutherford Crockett. Set in Scotland and France in the fifteenth century, it has both historical and supernatural elements. Historical figures include William Douglas and Gilles De Retz, while supernatural elements present in the novel are the werewolves. 

Tolkien might have been influenced in the following writing choices in the Lord of the Rings:

- the battle with the wargs in The Lord of the Rings was inspired by the battle with werewolves (as he confirmed in a letter);

- the character of Gilles De Retz might have influenced the dark lord Sauron;

- Sholto's secret mail-coat could resemble Frodo's mithril coat.